Britain's Home Secretary Theresa May speaks on the third day of the Conservative Party Conference in Manchester northern Britain, October 6 , 2015. REUTERS/Phil Noble

Britain's home secretary Theresa May arrives at Westminster Abbey for a thanksgiving service on the final day of 70th anniversary Victory in Europe (VE) day commemorations in central London May 10, 2015. REUTERS/Suzanne Plunkett

Britain's Home Secretary Theresa May curtsies as she greets Queen Elizabeth during a ceremonial welcome for Mexico's President Enrique Pena Nieto and his wife Angelica Rivera, at Horse Guards Parade in London March 3, 2015. The President and his wife are guests of Queen Elizabeth during their three day state visit to Britain. REUTERS/POOL/Leon Neal (BRITAIN - Tags: POLITICS ROYALS ENTERTAINMENT)

Britain's Prime Minister David Cameron (L) and Home secretary Theresa May arrive at a ceremonial welcome for the President of Singapore Tony Tan, and his wife, at Horse Guards Parade in London October 21, 2014. The President and his wife will be guests of Queen Elizabeth during the first state visit of a Singapore President to Britain.
REUTERS/Toby Melville (BRITAIN - Tags: POLITICS ROYALS)

Britain's Home Secretary Theresa May speaks as she opens the Ukraine Forum on Asset Recovery at Lancaster House in central London, April 29, 2014. REUTERS/Andrew Winning (BRITAIN - Tags: CRIME LAW POLITICS BUSINESS)

Britain's Home Secretary Theresa May gestures next to an unidentified man during the evening swimming session at the London 2012 Paralympic Games at the Aquatics Centre August 30, 2012. REUTERS/Suzanne Plunkett (BRITAIN - Tags: SPORT OLYMPICS SWIMMING POLITICS)

Britain's Home Secretary Theresa May arrives for the service of thanksgiving to mark the Diamond Jubilee of Britain's Queen Elizabeth at St Paul's cathedral in central London June 5, 2012. Four days of nationwide celebrations during which millions of people have turned out to mark the Queen's Diamond Jubilee conclude on Tuesday with a church service and carriage procession through central London. REUTERS/Kieran Doherty (BRITAIN - Tags: ANNIVERSARY ENTERTAINMENT SOCIETY ROYALS)

U.S. Secretary of State Hillary Clinton (R) meets with British Home Secretary Theresa May at the London Conference on Somalia, February 23, 2012. REUTERS/Jason Reed (BRITAIN - Tags: POLITICS)

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It identified them as Naa'imur Zakariyah Rahman, 20, of north London, and Mohammed Aqib Imran, 21, of south-east Birmingham.

Britain faces the most acute threat ever from Islamist militants seeking to inflict mass attacks, often with spontaneous plots that take just days to bring to execution, the head of the MI5 said in October.

After four militant attacks this year that killed 36 people in Britain - the deadliest spate since the London "7/7" bombings of July 2005 - MI5 chief Andrew Parker said the threat was at the highest tempo he had seen in 34 years of espionage.

Number 10 Downing Street is the official residence of the prime minister. It is heavily guarded.

In 1991, Irish Republican Army (IRA) militants launched a mortar bomb attack on Number 10. John Major, the prime minister at the time, was inside but not hurt.